1. Field of the Invention
The present invention relates to a thin film magnetic head, a method of fabricating the same, and a magnetic recording device, in particular to an induction-type thin film magnetic head used for a recording unit of a magnetic head with write and read elements which enables recording at a high recording density.
2. Description of Related Art
Accompanying increases in the recording densities of magnetic disk apparatuses, there is a need for thin film magnetic heads having sufficient recording ability on a recording medium imparted with a large coercive force. Such magnetic heads are required to be composed of a material having a high saturation magnetic flux density (Bs) as a magnetic core material. As alloys which have a high Bs of 2.0 T or more and which can be produced by a wet film formation process used in a head core production process, alloys such as Coxe2x80x94Fe and Coxe2x80x94Nixe2x80x94Fe are known. For example, in IEEE Trans. Magn., vol. 23, p. 2981, there is a description on a Coxe2x80x94Fe alloy having a Bs of 1.9 T for which film formation was carried out by an electroplating method. Further, a Coxe2x80x94Nixe2x80x94Fe ternary soft magnetic thin film is disclosed and suggested in, for example, Japanese Patent Application Laying-Open Nos. 6-89422 and 6-346202. In particular, Japanese Patent No. 2821456 discloses that a ternary alloy film comprising 40 to 70% by weight Co, 10 to 20% by weight Ni, and 20 to 40% by weight Fe, is formed from a plating bath free of a stress relaxation agent, thereby producing a soft magnetic thin film having a high Bs of 1.9 T or more. Furthermore, in xe2x80x9cFerromagnetismxe2x80x9d by R. M. Bozorth (D. Van Nostrand Co., Inc., N.Y., (1951) p. 441) there is a description on the physical properties of an Fexe2x80x94Co alloy having a high Bs which was produced by a sputtering method.
In order to realize a magnetic disk apparatus having a high recording density, a magnetic head core material with a high Bs should be used. In addition, in order to observantly keep pace with changes in induced current due to recording current, the coercive force (Hc) of the core material must be as small as possible. Moreover, as a recording track width becomes narrower, the aspect ratio of a resist frame tends to become larger. Thus, to uniform a film in the resist frame it is necessary to produce a magnetic head core using a wet film formation process, typically represented by plating.
A composition area having a high Bs in an alloy containing Co, Ni and Fe is an area having a body-centered cubic (bcc) structure as a crystal structure. However, in general, since magnetocrystalline anisotropy is large for an alloy of bcc phase, and the alloy tends to have a larger crystal grain size, and crystal grain size and Hc are strongly related due to crystal symmetry, it is impossible to obtain a sufficiently soft magnetic property suitable for a magnetic head.
Japanese Patent Application Laying-Open No. 2-69906 describes that a Coxe2x80x94Fe soft magnetic film of a bcc phase was manufactured by a sputtering method and that a good soft magnetic property of Hc=2Oe was obtained. However, because its Bs is low at 1.9 T it is inadequate for use as a recording head material designed for high-density recording. Further, while Japanese Patent Application Laying-Open No. 7-3489 describes a Coxe2x80x94Nixe2x80x94Fe alloy having a low Hc of 0.5 Oe or less, due to its composition containing a face-centered cubic (fcc) structure, it cannot be expected to have a high Bs.
As is clear from the above-mentioned known references, it has been difficult to select a material having a high Bs and small Hc that is applicable in a recording head core fabrication process. An object of the present invention is, accompanying the increasing recording densities of magnetic disk apparatuses, to provide a thin film magnetic head having sufficient recording performance on a recording medium imparted with a large coercive force, and a magnetic disk apparatus using the same.
Heretofore, there have been many reports which suggest that a composition area having a high Bs and a substantial bcc structure has insufficient soft magnetic properties.
According to the present invention, in an electroplating method, by applying a higher potential at an initial stage of the plating at a current density that is 5 times or more higher than usual and performing formation of a magnetic film with modulation of the potential at a film formation stage, it is possible to obtain a plated film having a crystal grain size modulated in the direction of film thickness, thus enabling reduction of Hc. According to this method, a magnetic thin film having a high Bs and an excellent soft magnetic property can be obtained.
It is generally known that the crystal grain of a plated film becomes fine by conducting plated film formation under a mass transfer rate-determining condition, such as in a method in which plating is performed at a high current density. However, as there are many crystal grain boundaries, a drawback exists in that corrosion resistance of a film cross section is inferior.
Further, even when films having different crystal grain sizes are laminated to perform the modulation of crystal grain size in the direction of film thickness, because of the formation of a plurality of layer boundary faces with different grain sizes in the plating, in a similar manner, the corrosion resistance of the film cross section is inferior, particularly with regard to a multi-layered film obtained by laminating a Coxe2x80x94Nixe2x80x94Fe plated film.
Therefore, by modulating a crystal grain size in the direction of film thickness and enhancing soft magnetic properties with fine crystal grains at an initial stage of plating, it is expected that, for a Coxe2x80x94Nixe2x80x94Fe film in a composition area of bcc phase, which has a large Hc that is likely to result in a rough crystal grain, the Hc can be reduced. Thus, it is possible to obtain a magnetic thin film having excellent soft magnetic properties and excellent corrosion resistance, while retaining a high Bs.
A thin film magnetic head according to the present invention comprises: a lower magnetic core; an upper magnetic core which is opposed to the lower magnetic core through a magnetic gap at a tip portion and connected to the lower magnetic core at a rear portion; and a coil magnetically connected to a magnetic circuit formed of the lower and upper magnetic cores. The thin film magnetic head is characterized in that at least a part of the lower and/or the upper magnetic core contains Co, Ni and Fe, and that the thin film magnetic head further comprises a plated magnetic film wherein the crystal grain size is modulated in the direction of film thickness. Herein, a structure wherein the crystal grain size is modulated in the direction of film thickness means a structure wherein the crystal grain size changes seamlessly or gradually towards the direction of a surface side from a substrate side of the plated film. A structure wherein a crystal shape changes can also be regarded as modulation of crystal grain size.
The plated magnetic film comprises not less than 25% by weight of Fe, not greater than 80% by weight of Co, and not greater than 20% by weight of Ni, and the crystal structure of the magnetic film is substantially a body-centered cubic structure. As can be seen from the relationship between Bs and composition of the Coxe2x80x94Nixe2x80x94Fe magnetic film shown in FIG. 1, the magnetic films in this composition area have a saturation magnetic flux density of 2.0 T or more.
The plated magnetic film can be formed on an underlayer comprising a sputtered film containing Co, Ni and Fe. The formation of a plated magnetic film on such a underlayer allows a magnetic property of a magnetic core to be improved.
The sulfur content of the plated magnetic film is preferably not greater than 0.1% by weight. If the sulfur content exceeds 0.1% by weight, Bs will be decreased.
A method of fabricating the thin film magnetic head according to the present invention comprises the steps of: forming a lower magnetic core on a substrate; forming a coil; and forming an upper magnetic core which is opposed to the lower magnetic core through a magnetic gap film at a tip portion, connected to the lower magnetic core at a rear portion, and is magnetically connected to the coil together with the lower magnetic core. The method is characterized in that in the steps of forming the lower magnetic core and/or forming the upper magnetic core, a plated magnetic film containing Co, Ni and Fe is formed such that crystal grains have a grain size modulated in the direction of film thickness.
The plated magnetic film can be formed by electroplating in which current value or potential is modulated continually, gradually or periodically. When direct current is used as plating current, the plating current density is from 50 to 1000 A/m2 and film formation is carried out under a plating current 5 times or more higher than the conventional magnetic film plating condition. In particular, at an initial stage of the plating process, a high plating potential is applied. Thereafter, the potential is modulated so as to become low, to thereby enable obtainment of a structure in which the crystal grain size is modulated in the direction of film thickness. However, this is only one example of such a method, and even if the crystal grain size is modulated by a method other than the above, it does not go against the subject-matter of the present invention.
A plating bath of electroplating may contain saccharin sodium as a stress relaxation agent. Saccharin sodium has a stress relaxation effect on a plated film while retaining a high Bs, and thus a high physical stability is obtainable.
The thin film magnetic head of the present invention is optimum for use as an induction-type magnetic recording head of a thin film magnetic head with write and read elements, in which a magnetoresistive element and an induction-type magnetic head are used as a read element and a write element, respectively.
According to the present invention, a magnetic film is formed by electroplating in which potential is modulated in a plated film formation process so that the crystal grain size of the magnetic film is modulated in the direction of film thickness, to thus enable the Hc of a Coxe2x80x94Nixe2x80x94Fe plated film in a bcc composition area to be reduced. Further, when a magnetic head is manufactured using this plated magnetic film, a magnetic disk apparatus having a high recording density can be obtained.